"You want to make sure you are being very focused on negotiating your best opportunities, because that will help determine what you're getting paid in future roles," certified financial planner Dawn Rapoport told Business Insider. "If you spend 10 years being undervalued, that doesn't set you up for the middle or later stages of your career, where you should hit peak earning power."

To get the offer you're looking for, avoid these common, yet costly, pitfalls:

1/

You don't ask

According to a Careerbuilder survey, a whopping 56% of workers have never asked for a raise, and women are less likely to ask than men. The same survey found that two-thirds of workers who ask for a raise get one, and the success rates are virtually the same for men and women who ask.

You can't sit around and expect a raise or bonus to fall into your lap. Even if your boss notices your hard work and efficiency, he or she won't necessarily pay you more. You have to be proactive and ask for what you want.

As personal-finance expert Farnoosh Torabi, who doubled her salary at 26, preaches, "You don't get what you deserve. You get what you negotiate."

You ask too soon

"Millennials got so many participation trophies growing up that a recent study showed that 40% believe they should be promoted every two years, regardless of performance," writes Time magazine's Joel Stein.

This mentality can be costly. As self-made millionaire Sophia Amoruso, who built her $100 million company, Nasty Gal, from scratch, writes in "#GIRLBOSS":

You need to get your hands dirty and spend time proving yourself before you ask for a raise or a promotion. Four months are not enough, and neither are eight. At the bare minimum, you need to be in your position for a year before you ask for a raise or title change.

Even then, that's if and only if you've been going above and beyond, doing work that's outside your job description, and generally making yourself completely indispensable to your employer.

3/

You don't come prepared

Asking is only half the battle. You can't simply say, "I want a raise" and not support your request. If you come prepared to highlight your accomplishments, skills, and experience, you can justify what you're asking for.

You should also look at the salary range for someone with your level of experience and in your industry or company. Glassdoor, PayScale, Salary.com, and Indeed.com all offer free compensation and benefits information.

As personal-finance expert Ramit Sethi says of salary negotiation, "80% of the work happens before you enter the room."

You've made it clear you wouldn't ever consider leaving

"If you approach a boss making it clear you're not going to leave, why should [they] pay you more?" he points out. "It's always about what you're worth in the market based on skills and experience. Either your current employer will give it to you, or you need to go out in the market."

You talk about what's 'fair'

Don't enter a negotiation seeking a "fair outcome," Miller explains: "The concept of fairness is not an effective way to approach negotiation of salary because it's going to put the employer on the defensive."

Rather, approach the negotiation as more of a contest, in which you're trying to get the best outcome possible. Two things will get you that outcome, Miller says: When you have another offer or when your employer is afraid you might seek another offer.

"When you say, 'My salary is not fair,' that engenders a negative response," he says. "If you say, 'I've been approached by so-and-so and they're talking about a job where I'm getting x, y, or z, no one views that as pushy. Their response is, 'I don't want to lose this person, so what can I do to fix this?'"